1. Field of the Invention
The present invention relates to polyurethane resin water dispersion and to aqueous polyurethane adhesive employing the polyurethane resin water dispersion.
2. Description of the Prior Art
Polyurethane resin is in wide use for applications to adhesive in terms of excellence in adhesion, flexibility and mechanical strength.
Organic solvent polyurethane resin in which polyurethane resin is dissolved in organic solvent has been generally used as the polyurethane adhesive. However, the organic solvent does not necessarily have a good influence on human body and environment and involves a risk of catching fire. In view of this, as a substitution for this organic solvent polyurethane adhesive, aqueous polyurethane adhesive comprising polyurethane resin water dispersion in which polyurethane resin is dispersed in water is now developing with great speed, with increasing interests to the environmental sanitation in recent years.
For example, Japanese Laid-open (Unexamined) Patent Publication No. Sho 63 (1988)-69882 proposes as an example of the aqueous polyurethane adhesive self-emulsifiable aqueous polyurethane adhesive produced by the reaction of aliphatic polyisocyanate with adipic-acid polyester polyol and polyol having carboxylic acid group or sulfonic acid group.
Japanese Laid-open (Unexamined) Patent Publication No. Hei 9 (1997)-157625 proposes, for example, in its example 4 that 3-isocyanatomethyl-3,5,5,-trimethylcyclohexyl isocyanate is allowed to react with polycaprolactonediol having a number average molecular weight of 2,000 and dimethylolpropionic acid to synthesize isocyanate terminated prepolymer, first, and, then, the isocyanate terminated prepolymer thus synthesized is allowed to react with ethylene diamine in water, to prepare polyurethane resin water dispersion, for use as the aqueous polyurethane adhesive.
The polyurethane resin water dispersion comprising adipic-acid polyester polyol as proposed by Japanese Laid-open (Unexamined) Patent Publication No. Sho 63 (1988)-69882 cited above is excellent in adhesion and thus is in general use. However, the polyurethane resin water dispersion comprising adipic-acid polyester polyol has the disadvantages that it is low in heat resistance at temperatures exceeding crystalline melting point; that the ester linkage of adipate is so susceptible to hydrolysis that the storage stability and the adhesion performance after storage of the water dispersion can easily deteriorate; and that even after adhesive bonding, significant performance deterioration due to hydrolysis is incurred in the presence of moisture, particularly at temperatures exceeding crystalline melting point.
On the other hand, the polyurethane resin water dispersion comprising polycaprolactone polyester polyol as proposed by Japanese Laid-open (Unexamined) Patent Publication No. Hei 9 (1997)-157625 cited above has good heat resistance and the advantage that the ester linkage of caprolactone being hardly susceptible to hydrolysis. Accordingly, it has good water resistance, storage stability for water dispersion, and adhesion performance after storage, as compared with the polyurethane resin water dispersion comprising adipic-acid polyester polyol mentioned above. On the other hand, it has the disadvantage of being low in initial adhesion so that it cannot provide any adhesion force able to bring base material into destruction in a short time, as the organic solvent polyurethane resin adhesive, for example.